CSE 143 Lecture 6 Interfaces; Complexity (Big-Oh) reading: 9.5, 11.1, 13.1 - 13.3 slides created by Marty Stepp http://www.cs.washington.edu/143/

Related classes Consider the task of writing classes to represent 2D shapes such as Circle, Rectangle, and Triangle. Certain operations are common to all shapes: perimeter: distance around the outside of the shape area: amount of 2D space occupied by the shape Every shape has these, but each computes them differently.

Shape area and perimeter Circle (as defined by radius r ): area =  r 2 perimeter = 2  r Rectangle (as defined by width w and height h ): area = w h perimeter = 2w + 2h Triangle (as defined by side lengths a, b, and c) area = √(s (s - a) (s - b) (s - c)) where s = ½ (a + b + c) perimeter = a + b + c r w h a b c

Common behavior Suppose we have 3 classes Circle, Rectangle, Triangle. Each has the methods perimeter and area. We'd like our client code to be able to treat different kinds of shapes in the same way: Write a method that prints any shape's area and perimeter. Create an array to hold a mixture of the various shape objects. Write a method that could return a rectangle, a circle, a triangle, or any other kind of shape. Make a DrawingPanel display many shapes on screen.

Interfaces (9.5) interface: A list of methods that classes can promise to implement. Inheritance gives you an is-a relationship and code sharing. A Lawyer object can be treated as an Employee, and Lawyer inherits Employee's code. Interfaces give you an is-a relationship without code sharing. A Rectangle object can be treated as a Shape but inherits no code. Analogous to non-programming idea of roles or certifications: "I'm certified as a CPA accountant. The certification assures you that I know how to do taxes, perform audits, and do consulting." "I'm a Shape. I know how to compute my area and perimeter."

Interface syntax public interface name { public type name(type name, ..., type name); ... } Example: public interface Vehicle { public int getSpeed(); public void setDirection(int direction);

Shape interface abstract method: A header without an implementation. // Describes features common to all shapes. public interface Shape { public double area(); public double perimeter(); } Saved as Shape.java abstract method: A header without an implementation. The actual bodies are not specified, because we want to allow each class to implement the behavior in its own way.

Implementing an interface public class name implements interface { ... } A class can declare that it "implements" an interface. The class promises to contain each method in that interface. (Otherwise it will fail to compile.) Example: public class Bicycle implements Vehicle {

Interface requirements public class Banana implements Shape { // haha, no methods! pwned } If we write a class that claims to be a Shape but doesn't implement area and perimeter methods, it will not compile. Banana.java:1: Banana is not abstract and does not override abstract method area() in Shape ^

Interfaces + polymorphism Interfaces benefit the client code author. They allow client code to take advantage of polymorphism (the same code is able to work with different types of objects). public static void printInfo(Shape s) { System.out.println("The shape: " + s); System.out.println("area : " + s.area()); System.out.println("perim: " + s.perimeter()); System.out.println(); } Any shape can be passed as the parameter to the method. Circle circ = new Circle(12.0); Triangle tri = new Triangle(5, 12, 13); printInfo(circ); printInfo(tri);

ADTs as interfaces (11.1) abstract data type (ADT): A specification of a collection of data and the operations that can be performed on it. Describes what a collection does, not how it does it. Java's collection framework describes ADTs with interfaces: Collection, Deque, List, Map, Queue, Set, SortedMap An ADT can be implemented in multiple ways by classes: ArrayList and LinkedList implement List HashSet and TreeSet implement Set LinkedList , ArrayDeque, etc. implement Queue They messed up on Stack; there's no Stack interface, just a class.

Using ADT interfaces It is considered good practice to always declare collection variables using the corresponding ADT interface type: List<String> list = new ArrayList<String>(); Methods that accept a collection as a parameter should also declare the parameter using the ADT interface type: public void stutter(List<String> list) { ... }

Why use ADTs? Why would we want more than one kind of list, queue, etc.? Answer: Each implementation is more efficient at certain tasks. ArrayList is faster for adding/removing at the end; LinkedList is faster for adding/removing at the front/middle. HashSet can search a huge data set for a value in short time; TreeSet is slower but keeps the set of data in a sorted order. You choose the optimal implementation for your task, and if the rest of your code is written to use the ADT interfaces, it will work.

Algorithm growth rates (13.2) We measure runtime efficiency not in seconds, but in proportion to the input data size N. growth rate: Change in runtime as N changes. Say an algorithm runs 0.4N3 + 25N2 + 2N + 17 statements. Consider the runtime when N is extremely large. We ignore constants like 25 because they are tiny next to N. We only look at the highest-order term (N3) because it dominates. We say that this algorithm runs "on the order of" N3. or O(N3) for short ("Big-Oh of N cubed")

Complexity classes complexity class: A category of algorithm efficiency based on the algorithm's relationship to the input size N. Class Big-Oh If you double N, ... Example constant O(1) unchanged 10ms logarithmic O(log2 N) increases slightly 175ms linear O(N) doubles 3.2 sec log-linear O(N log2 N) slightly more than doubles 6 sec quadratic O(N2) quadruples 1 min 42 sec cubic O(N3) multiplies by 8 55 min ... exponential O(2N) multiplies drastically 5 * 1061 years

Collection efficiency Efficiency of various operations on different collections: Method ArrayList SortedIntList Stack Queue add (or push) O(1) O(N) add(index, value) - indexOf O(?) get remove set size Method ArrayList SortedIntList Stack Queue add (or push) add(index, value) indexOf get remove set size

Binary search (13.1, 13.3) binary search: An algorithm that searches a sorted array or list by successively eliminating half of the elements. Examine the middle element of the array. If it is too big, eliminate the right half of the array and repeat. If it is too small, eliminate the left half of the array and repeat. Else it is the value we're searching for, so stop. Which indexes does the algorithm examine to find value 22? What is the runtime complexity class of binary search? index 1 2 3 4 5 6 7 8 9 10 11 12 13 14 value -4 -1 22 29 31 37 56